The Soil P Cycle

In the soil, not all phosphorus is the same. It can be a part of organic molecules or part of inorganic molecules. In addition, the chemicals that contain P will change in the soils. Therefore, it is important to think about the P cycle in the soil (Fig. 6). The availability of soil P to plants is dependent on the reactions of different chemical forms of soil P. Phosphorus inputs to the soil are primarily from the application of fertilizer P and organic resources which contain P, such as manure. Lesser amounts may be added due to deposition from the atmosphere and sedimentation. Soil P is generally categorized into three types: solution Plabile P, and non-labile P (Fig. 6).

Figure 6.  The soil P cycle. (Image from Sharpley and Sheffield, Livestock and Poultry Environmental Stewardship Curriculum)

Solution P is a very small part of the total soil P, but is the P fraction taken up by plants; and, if carried in runoff water, may result in immediate stimulation of aquatic growth. Labile soil P is more plentiful than soluble P, but is still only a small fraction of total soil P. Labile P is not strongly adsorbed in the soil and may enter the soluble phase relatively quickly. Lastly, stable, or non-labile, P is in forms unavailable to plants and constitutes the greatest fraction of total soil P. With time, a small amount of non-labile P reacts chemically to become labile P and soluble P. Most non-labile P will remain in the non-labile form indefinitely.

Discussion Question: Which soil P fraction is likely to be most affected immediately after manure application?

Answer: Solution P as much manure P is soluble and the amount applied is quite large relative to the quantity of solution P.

Phosphorus at the soil surface is exposed to movement in runoff and erosion. If farmers do not practice deep tillage, applied P becomes concentrated in the surface layer of soil. When P is surface-applied in fertilizer and manure, there is potential for much soluble P loss during the days following application, since the concentration of soluble P at the soil surface is greatly increased.

With time, the potential for soluble P loss decreases rapidly after application.  This is due to the adsorption of soluble P to surfaces of soil minerals and organic matter, with iron and aluminum oxides, and with calcium carbonate in calcareous soils (Fig. 7). In addition, much of the P can be adsorbed to soil aggregates and not easily moved by erosion, resulting in less runoff loss of labile and non-labile P. Increased macro-aggregate formation is another effect of applied manure that reduces the potential for P loss with time after application. Bits of the organic material in manure provide nuclei that attract clay and silt particles (Fig. 8).

Figure 7.  As P is absorbed into the soil after application, the risk of dissolved P being lost to runoff is reduced over time. (Image courtesy of Charles Wortmann)

Figure 8. Macro-aggregate formation and stability are often improved with manure application, resulting in much applied P being protected from runoff in aggregates. (Image courtesy of Charles Wortmann)

The added organic material also stimulates microbial activity. Bacteria may give off mucilage that has an adhesive effect on soil aggregates. The mycelia of fungi also work to bind particles and micro-aggregates (< 0.25-mm diameter) together to form macro-aggregates (> 0.25-mm diameter). More aggregation results in better water infiltration, less runoff and erosion, and less P loss.